This invention relates generally to multi-dimensional magnetic resonance imaging (MRI), and more particularly the invention relates to MRI reconstruction in the presence of multiple image signal distortions.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI signals for reconstructing an image of an object are obtained by placing the object in a magnetic field, applying magnetic gradients for slice selection, applying a magnetic excitation pulse to tilt nuclei spins in the desired slice, and then detecting MRI signals emitted from the tilted nuclei spins while applying readout gradients. The detected signals can be envisioned as traversing lines in a Fourier transformed space (k-space) with the lines aligned and spaced parallel in Cartesian trajectories or emanating from the origin of k-space in spiral trajectories.
Self-navigated interleaved spirals (SNAILS) have been used for high resolution diffusion weighted imaging (DWI). Although spiral trajectories have many advantages for fast image acquisition, they normally suffer from image blurring caused by off-resonant spins. Many techniques have been developed for off-resonance correction. However, few studies have been reported for off resonance correction for multi-shot DWI. One difficulty in this situation originates from the k-space data distortion caused by motion-induced phase errors.
Diffusion-weighted imaging (DWI) provides a unique tissue contrast by sensitizing random molecular thermal motion using magnetic field gradients. Because of its ability to quantify this random motion, DWI has become a powerful tool for studying tissue micro-structures and detecting acute ischemic stroke in which diffusion is highly restricted very early after the onset of stroke.
Patient motion during diffusion encoding results in additional phase terms that lead to severe ghosting artifacts if not accounted for. To avoid such artifacts, diffusion-weighted images are therefore most commonly acquired using single-shot echo planar imaging (EPI) sequences. The drawbacks of single-shot EPI include relatively low image resolution limited by T2* decay and geometric distortions caused by magnetic field inhomogeneity. One technique to overcome the aforementioned distortions is combining sensitivity encoding (SENSE) with single-shot EPI. Alternatively, several multi-shot sequences, for example PROPELLER and SNAILS (self-navigated interleaved spiral), have been shown to be capable of acquiring high resolution (up to 512×512) diffusion-weighted images with significantly diminished distortions.
One common problem inherent to multi-shot diffusion-weighted image acquisition is image-domain phase perturbation that varies from shot to shot, which has thus far limited the application of parallel imaging to multi-shot DWI. This phase variation can be either linear or non-linear. The linear phase variation is usually caused by rigid-body motion during diffusion encoding periods; whereas the non-linear phase can be caused by nonrigid motion, for example by brain pulsation. Correcting this phase variation by subtracting a low resolution phase map from each shot has proven efficient. The phase map can be obtained either from an extra navigator image or from the k-space data of a self-navigated trajectory (e.g. variable density spirals). This simple phase correction algorithm can remove effectively the phase error to a certain degree which has been applied in both PROPELLER and SNAILS DWI. This algorithm was thought to be adequate, provided that each shot samples the k-space above the Nyquist rate. PROPELLER DWI is one technique that satisfies this sampling criterion. More specifically, in PROPELLER DWI, during each shot a selected segment of the k-space is sampled at the Nyquist rate. Data acquired from each shot results in a unaliased, but blurred, image, which permits a straight-forward phase subtraction.
Although successful image reconstruction has been demonstrated with PROPELLER DWI, the phase variation cannot be completely corrected by the direct subtraction algorithm due to image blurring or aliasing. For PROPELLER DWI, because the phase error is usually smooth, the residual artifact might not be so severe. For arbitrary, undersampled k-space trajectories, however, the incomplete phase correction worsens considerably because of possible aliasing artifacts. For example, when each shot undersamples some portions of the k-space, the effect of aliasing causes the phase error at one location to appear at other locations. The resulting non-localized phase error can no longer be corrected through a simple phase subtraction. Therefore, images reconstructed using this simple phase subtraction algorithm may still suffer from severe motion-induced artifacts and residual ghosts may have to be suppressed using multiple averages.
The present invention is directed to a generalized iterative image reconstruction which simultaneously corrects for multiple signal distortions including phase, off resonance and gradient non-linearities.